Indicator for chloride titrations



United States PatentO INDICATOR FOR CHLORIDE TITRATIONS Frank E. Clarke,West Annapolis, Md.

No Drawing. Application July 20, 1955, Serial No. 523,392

19 Claims. (Cl. 23-230) (Granted under Title 35, U. S. Code (1952), see.266) The invention described herein may be manufactured an'd'used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention relates to a chemical indicator utilizable in and amethod for the quantitative determination of the chloride ion in waterand aqueous solutions generally.

This application is a continuation-in-part of application Serial No.155,774, filed April 13, 1950, now abandoned.

There are known and used several methods for measuring the chloride ionin solutions, the most important of which may be mentioned as follows:(a) the Mohr method involving the titration of a neutral chloridesolution with silver nitrate solution, using potassium chromate solutionas an end point indicator; (b) the precipitation of chloride as silverchloride with silver nitrate and weighing the precipitate; (c) theaddition of silver nitrate solution to a nitric acid acidified solutionof the chloride and comparison of the turbidity formed with standardturbidities developed in the same manner; and (d) the Volhard method ofadding excess silver nitrate to acidified chloride solution andtitrating the silver in excess of the silver-chloride equivalence pointwith thiosulphate solution.

These methods, while useful, have certain disadvantages. For example, inthe Mohr (a) method the silver nitrate-potassium chromate titration isbased on a subtle change of solution color from yellow to orange red atthe silver-chloride equivalence'point. This change occurs gradually andthe true end point" cannot be detected readily in solutions containinglow concentrations of chloride v(20 parts per million or less). Even inhigher concentrations, satisfactory titrations can be made only by ahighly skilled technologist.

In the (b) method, the gravimetric determination of chloride as silverchloride is extremely slow and involves many analytical manipulations,such as filtering and weighing, and is not'p'r'acticable for lowconcentrations of chlorine (20 parts per million or'less).

The method is restricted to clear uncolored waters and to chlorideconcentrations below ten parts per million. It islcumbersome and theresults are somewhat ambiguous because they depend on matching ofturbidities rather than true colors.

The Volhard titration (d) is restricted by the fact that it does nothave a sharp end point. This is because silver thiocyanate graduallyreacts with silver chloride after all of the free silver ions havereacted with the thiocyanate ions.

In addition to those mentioned, two additional methods have beenproposed, the one involving the titration of a nitric acid acidifiedsolution of chloride with mercuric nitrate solution using eitherdiphenylcarbazide or diphenylcarbazone indicator; and the otherinvolving'titration of a nitric acid "acidified solution of chloridewith mercuric nitrate, using bromophenol blue as a pH indicator anddiphenylcarbazide as the end point indicator.

2,784,064 cg '3 f ast???it??? 2 .1 These latter proposals have inherentdeficiencies in that titrations according to the first proposal sufferfrom the deficiency of premature development of end pointc'olor, and inaddition the titrations with diphenylcarbazide indicator have theaddeddeficiencies of instability of end point indicator and low contrastof the fully developed end point color. Titrations according totheseco'nd proposal have deficiencies of instability of end pointindicator and low contrast of end point color particularly in the lowchloride concentrations. In addition the useof a separate pH indicatoris a disadvantage, particularly in field Work, and this disadvantage isaccentuated by the fact that the color change point of the pH indicatoroccurs at approximately pH 3.5 while the optimum pHfor thediphenylcarbazide titration is approximately 2,0.

The general and primary object of my invention is to overcome thedeficiencies of the prior methods and pro posals in a simple andsatisfactory titration procedure.

Additionally, objects of the invention'include a'method of chloride iondetermination which has extreme sensitivity with'sharp and-pronounceddiscernability; which has pronounced accuracy; which operates with ahigh degree of speed and lack of complication; and which is generallyinditferent to interference of other type ions and to turbidity andcolor. 1 An object of outstanding importance 'also'is the pro vision ofan indicator which combines in one single stable solution both apotentially sensitive and discernible end point indicator and a pHindicator, capable at the same time of facilitating simultaneously. theadjustment of the optimum pH for the titration and of providing an efiective background color for detection of the end point.

A preferred form of indicator in accordance with the invention consistsof pure diphenylcarbazone (CsHsNNCONHNHCsI-Is) and pure crystallinetetrabromo phenol sulphonphthalein (C19H10B14O5S) (bromo phenol blue)dissolved in alco hol, preferably methyl or ethyl alcohol. Preferablyalso an optimum concentration is used containing 0.5 gram to 1.0 gram byweight of the diphenylcarbazone and 0.05 gram to 0.10 gram of the bromophenol blue in solution in suflicient pure ethyl alcohol or pure methylalcohol to make milliliters of solution. This means that the aliphaticalcoholic solu tion contains asolute having diphenylcarbazone in apreferred range of 83-95% and bromo phenol blue in a preferred range of4.8-17 by weight of solute. (Com putations to two significant figures.)A proficient indicator is-obtained with .5 gram diphenylcarbazone and.05 gram bromo phenol blue.

While the concentration mentioned is desirable, it-may be varied to meetspecial requirements to the limit'of solubility of both ingredients,that is, about 2 grams 'of diphenylcarbazone and about 0.5 gram bromophenol blue per 100 milliliters of alcoholic solution. Considered withthe previously stated weights of 0.5 gram of diphenylcarbazone and .05gram of bromo phenol blue, this means that the solute approximately hasdiphenylcarbazone in a range of 50-98% and bromo phenol blue in a rangeof 2.450% by weight. However, obviously such change from the preferredranges of concentration would affect the discernibility and accuracy ofthe indicator andpo'se sibly require correction by change in thequantity of indi-'. cator used in the titration.

The two ingredients of the alcohol solution, namely,

diphenylcarbazone and bromo phenol blue, constitute the. indicator unitfor the quantitative determination of the minant and the bromophenol-blue both as a pH indicator and to mask the premature developmentof end color by the diphenylcarbazone.

In use as, for example, in the titration of chloride ion .with amercuric nitrate reagent, 'five drops of the red indicator fluid areadded to a sample (generally 25-100 milliliters) of the solution to betested. The pH of the solution is then adjusted with nitric acid, orwith an alkali followed by the acid, to just obtain the yellow acidcolor of the bromo phenol blue component, and a slight excess of nitricacid (1 ml. of 0.05 N nitric acid per 100 ml. of samplesolution) is thenadded. Mercuric nitrate solution (generally 0.025 N to 0.1 N) is nowadded, dropwise, until the yellow color oi the solution changes toblue-violet at the mercury-chloride equivalence point, thus completingthe determination.

'The highly important and useful characteristics of the indicator unitas above described may now be delineated.

It is noted that from experimental data available, the optimum pH forthe diphenylcarbazone indication of the chloride reaction isapproximately 3.3. Since the color change point of the bromo phenol blueconstituent is pH 3.5 to 3.6, it is apparent that acceptable accuracyisobtainable by the simple procedure of adding nitric acid slightly inexcess of this color change point and then proceeding with-the mercuricnitrate-chloride titration. The purpose of the nitric acid addition isto insure highest accuracy by establishing approximately the optimum pHvalue of 3.3. p

The diphenylcarbazone alone would develop a creeping endpoint. By addingthe bromo phenol blue, not only is the pH value indicated, but thepremature end point is masked so that the change from pale yellow (acid)to intense blue-violet occurs in the sensitive range of one drop of0.025 normal mercuric nitrate solution. By this means marked contrastand discernibility are secured and hence accurate detection of themercury-chloride equivalence point is made possible.

The sensitivity and versatility of the indicator is outstanding,chloride concentrations from a fraction of a part per million tothousands of parts per million being accurately indicated. Further,because the indicator mixture contains indicators for both adjustment ofpH and detection of end point, it allows simpler and fasterdetermination of chloride than has heretofore been possible, with thepossible exception of the straight mercuric nitratediphenylcarbazide andmercuric nitrate-diphenylcarba zone-titrations. These latter titrations,however, as previously indicated are not reliable.

A further advantage of the indicator is its availability for usefulperformance with all ions apt to be encountered in water and solutionswith the exceptions of chromate and ferric. However, both of these lastmentioned ions may be eliminated. The indicator also, because of itscontrasting endpoint color, functions well in turbid and highly coloredsolutions,.and even in bluesolutions.

V Ajfurther important characteristic of the indicator is that'itpossesses a high degree .ofstability. Shelf tests of as. much as sixmonths have produced no effects on the indicating capability of theindicator.

Tests have shown that usable modifications of the. titration proceduremay be made. For example, instead of using amixture-or a solution of theingredients, the indicator elements may be added separately to the testsolution, Further, thesodium salt of .bromo phenol blue in watersolution mightbe substituted for bromo phenol blue. in alcohol solution.In the former case,.i. e., separateadditions of the indicatorcomponents,,either com ponent can be added first or-both addedsimultaneously. The indicator unit canalso be usedwithother aliphaticalcohols, for example propyl, butyl, amyland ,hexyl, these specificalcohols containing up to six carbon atoms. However, the indicatoringredients donothaveas high maximum solubilities in the higheralcohols, the maximum solubility of the diphenylcarbazone and bromophenol 4 blue being about 1.5 grams and .2 gram, respectively, in thesealcohols. Similarly,..the lower ranges of the, indicator ingredients canbe somewhat less than that stated hereinbefore.

Modification may also be made by substituting for the bromo phenol bluea compatible soluble dye, such as a mixture of Pontamine Fast Blue 4GLand Pontacyl Light Yellow 36 for increasing the discernibility of. thediphenylcarbazone end point. The Rowe Color Index and Supplement liststhe former dye under ,Color Index Number 533, and lists the latter underColor Index Number 636. This publication is put out by the Society ofDyers and Colorists, Bradford, Yorkshire, England; the index being dated1924 and the supplement 1928. Since this dye serves only to. producebackground or masking effect, it would be necessary to employ electronicpH measurements. T he drawback here is that all the dyes that improvethe sharpness of the diphenylcarbazoneend point reduce the accuracy ofthe titration and further add to the complexity and cost of theprocedure by requiring use of expensive and cumbersome electronic pHapparatus.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is: r

1. An indicator characterized by the ability to indicate both theoptimum pH.and the mercury-chloride equivalence point in the titrationof an aqueous chloride solution with mercuric nitrate, comprisingdiphenylcarbazone in a range of 50-98% and bromo phenol blue in a rangeof 2.4-50%, by weight.

2. An indicator characterized by the ability to indicate both theoptimum pH and the mercury-chloride equivalence point in the titrationof an aqueous chloride solution with mercuric nitrate, comprisingdiphenylcarbazone in a range of 83-95% and bromo phenol blue in a rangeof 4.8-17%, by weight.

3. An indicating solution for titration indication of chloride inaqueous solution, consisting essentially of a lower aliphatic alcoholcontaining from one to six carbon atoms, including as a solute 50-98%diphenylcarbazone and 24-50% bromo phenol blue,'by weight of solute.

4. An indicating solution for titration indication of chloride inaqueous solution, consisting essentially of a lower aliphaticalcoholcontaining from one to six carbon atoms, including as a solute83-95% diphenylcarbazone and 4.8-17% bromo phenol blue, by weight ofsolute.

5. An indicating solution as defined in claim 3 but characterized bysaid aliphatic alcohol being chosen from the class consisting of methyland ethyl alcohol.

6. An indicating solution as defined in claim 4 but characterized bysaid aliphatic alcohol being chosen from the class consisting of methyland ethyl alcohol.

7. A method for the determination of the chloride ion content in anaqueous solution, comprising adding as indicators bromo phenol blue anddiphenylcarbazone to a sample of the aqueous solution, thediphenylcarbazone serving as an end color determinant, and the bromophenol blue serving both as a pH indicator and to mask prematuredevelopment of end color by the diphenylcarbazone, adjusting the pH ofthe solution to just obtain the acid color of the bromo phenol blue, andtitrating with mercuric nitrate solution until thecolor of the aqueoussolution changes to a blue-violet.

8. A method as defined in claim 7 but further characterized by thediphenylcarbazone being about 50-98 percent and the bromo phenol bluebeing about 2.4-50 percent of their total weight.

9. A method as defined in claim ,8 butfurtherlqharacte'rized by saiddiphenylcarbaione beingadded in solution with a lower aliphatic alcoholcontaining from one to six carbon'atorns.

10. A method as defined in claim 9 but further characterized by saidalcohol being chosen from the class consisting of methanol and ethanol.

11. A method for the determination of the chloride ion content in anaqueous solution, comprising adding as indicators bromo phenol blue anddiphenylcarbazone to a sample of the aqueous solution, thediphenylcarbazone serving as an end color determinant and the bromophenol blue serving both as a pH indicator and to mask prematuredevelopment of end color by the diphenylcarbazone, adding nitric acid tothe aqueous solution to obtain the acid color of the bromo phenol blueand a pH of about 3.3, and titrating with mercuric nitrate.

12. A method as defined in claim 11 but further characterized by thediphenylcarbazone being about 50-98 percent and the bromo phenol bluebeing about 2.4-50 percent of their total weight.

13. A method as defined in claim 12 but further characterized by saiddiphenylcarbazone being added in solution with a lower aliphatic alcoholcontaining from one to six carbon atoms.

14. A method as defined in claim 13 but further characterized by saidalcohol being chosen from the class consisting of methanol and ethanol.

15. An indicator for titration procedures consisting of a solution ofingredients in proportions equivalent to 0.5 gram to 1.0 gram ofdiphenylcarbazone and 0.05 gram to 0.1 gram of crystalline tetrabromophenol sulphonphthalein in 25-100 milliliters of 95% aliphatic alcoholcontaining from one to six carbon atoms.

16. An indicator for titration procedures consisting of a solution ofingredients in proportions equivalent to 0.5 gram to 1.0 gram ofdiphenylcarbazone and 0.05 gram to 0.1 gram of crystalline tetrabromophenol sulphonphthalein in 25-100 milliliters of methyl alcohol.

17. An indicator for titration procedures consisting of a solution ofingredients in proportions equivalent to 0.5 gram to 1.0 gram ofdiphenylcarbazone and 0.05 gram to 0.1 gram by weight of crystallinetetrabromo phenol sulphonphthalein in 25-100 milliliters of ethylalcohol.

18. A step in the process of determining the chloride ion content of asolution which comprises adding to a measured quantity of this solutionfive drops of indicator formed of a mixture of diphenylcarbazone andtetrabromo phenol sulphonphthalein and an aliphatic alcohol solvent, thealcohol containing from one to six carbon atoms, adjusting the pH of thesolution to obtain the acid color of the tetrabromo phenolsulphonphthalein component, and adding mercuric nitrate solution untilthe solution color changes.

19. A step in the process of determining the chloride ion content of asolution which comprises adding to a measured quantity of the solutionfive drops of indicator formed of a mixture of diphenylcarbazone andtetrabromo phenol sulphonphthalein in 10 to 1 ratio and an aliphaticalcohol solvent, the alcohol containing from one to six carbon atoms,adjusting the pH of the solution with nitric acid to obtain the yellowacid color of the tetrabromo phenol sulphonphthalein component, andadding mercuric nitrate solution in drop increments until the yellowacid solution color changes to blue-violet.

References Cited in the file of this patent UNITED STATES PATENTS2,385,471 Scharer Sept. 25, 1945 2,599,697 Conklin June 10, 19522,617,715 Haller Nov. 11, 1952 2,643,230 Mooradian et al. June 23, 19532,663,692 Corso et al. Dec. 22, 1953

1. AN INDICATOR CHARACTERIZED BY THE ABILITY TO INDICATE BOTH THEOPTINUM PH AND THE MERCURY-CHLORIDE EQUIVALENCE POINT IN THE TITRATIONOF AN AQUEOUS CHLORIDE SOLUTION WITH MERCURIC NITRATE, COMPRISINGDIPHENYLCARBAZONE IN A RANGE OF 50-98% AND BROMO PHENOL BULE IN A RANGEOF 2.4-50%, BY WEIGHT.